The study investigates the quantitative and stoichiometric analysis of microRNAs (miRNAs) in exosomes, which are small vesicles secreted by various cell types and found in bodily fluids. Exosomes have been proposed to serve as vehicles for miRNA-based intercellular communication and as potential biomarkers in bodily fluids. However, the actual content of miRNAs in exosomes has not been well characterized. The researchers found that exosome fractions from plasma, a major source of miRNAs, contained only a small minority of the total miRNA content. They quantified the number of exosomes and miRNA molecules in samples from five diverse sources: plasma, seminal fluid, dendritic cells, mast cells, and ovarian cancer cells. On average, there was less than one molecule of a given miRNA per exosome, even for the most abundant miRNAs. This suggests that most individual exosomes do not carry biologically significant numbers of miRNAs and are unlikely to function as effective vehicles for miRNA-based communication. The findings challenge current models of exosome-mediated miRNA communication and highlight the need for revised mechanistic models. The study also discusses the implications for the use of exosomes as biomarkers and the potential for other populations of extracellular vesicles to carry miRNAs.The study investigates the quantitative and stoichiometric analysis of microRNAs (miRNAs) in exosomes, which are small vesicles secreted by various cell types and found in bodily fluids. Exosomes have been proposed to serve as vehicles for miRNA-based intercellular communication and as potential biomarkers in bodily fluids. However, the actual content of miRNAs in exosomes has not been well characterized. The researchers found that exosome fractions from plasma, a major source of miRNAs, contained only a small minority of the total miRNA content. They quantified the number of exosomes and miRNA molecules in samples from five diverse sources: plasma, seminal fluid, dendritic cells, mast cells, and ovarian cancer cells. On average, there was less than one molecule of a given miRNA per exosome, even for the most abundant miRNAs. This suggests that most individual exosomes do not carry biologically significant numbers of miRNAs and are unlikely to function as effective vehicles for miRNA-based communication. The findings challenge current models of exosome-mediated miRNA communication and highlight the need for revised mechanistic models. The study also discusses the implications for the use of exosomes as biomarkers and the potential for other populations of extracellular vesicles to carry miRNAs.